CodeWorld is an educational environment using Haskell. It provides a simple mathematical model for geometric figures, animations, and interactive and multi-player games. The web-based programming environment supports this educational mission with a zero-setup editor and compiler, easy sharing and cloud storage of projects, and the ability to run programs right in the web browser using GHCJS.CodeWorld is stable and has been used in schools for years! See the users page for a partial list. We're constantly improving the environment, though. Breaking changes, when necessary, are scheduled to occur between typical (U.S.) K-12 school semesters, to minimize disruption of existing classes.